The increase in the force of myocardial contraction is ultimately due to one of two mechanisms: an enhanced intracellular free Ca 2+ concentration (Cai) or an increased Ca 2+ sensitivity of the contractile proteins. Virtually all the traditional interventions do alter the amplitude and/or the time course of Cai transient. The value of these cardiotonic agents is severely limited by undesired effects. Among the new positive inotropic substances the thiadiazinone derivative, EMD 53998 (designed by E. Merck, Darmstadt, Germany) has a mixed action. In skinned myocardial fibers it has a potent effect to increase peak force and to shift leftward the pCa- force relationship, and in cell homogenates exhibits phosphodiesterase (PDE) inhibitory activity. However, the potency of myofilament sensitization relative to that of PDE inhibition is greater than for any substance available to date. We tested the effect of EMD 53998 and of its enantiomers (EMD 57033 and EMD 57439) in intact guinea pig cardiac cells, bathed in Hepes buffer, at room temperature and loaded with the fluorescent Ca 2+ indicator, Indo-1. Our aim was to ascertain whether optical enantiomers, the (+), EMD 57033 and the (-), EMD 57439 could stereoselectively separate the effect mediated through PDE inhibition from that obtained via an increased myofilament responsiveness to Ca 2+. The results show that all three substances exert a pronounced increase in twitch amplitude accompanied by parallel changes in velocity of shortening. The Cai transient was increased by EMD 53998 and EMD 57439; the (+)- enantiomer, on the contrary, did not change either the systolic or the [systolic-diastolic] Ca 2+ levels. These results indicate that the novel (+) enantiomer behaves as a pure myofilament Ca 2+ sensitizer in intact cells and may have therapeutic potential in some forms of heart failure.